1. Field of the Invention
The present invention relates to a heat roller type fixing apparatus provided in an electronic photographing apparatus and a method of supporting a roller shaft of a pressing roller provided in the fixing apparatus.
2. Description of the Related Art
In an electronic photographing apparatus, an image is formed on a recording paper sheet while a photosensitive drum is rotated 360.degree. according to the following process. First, a photo-sensitive face of the drum is electrified by an electrifier. Then, a light signal is radiated on the photo-sensitive face by an exposing device so that an electroslatic latent image is formed on the photo-sensitive face. Then, toner is adhered to an electrostatic latent image on the drum by means of a developing apparatus. The toner image formed on the drum is transferred onto the recording paper sheet by a transfer apparatus. Finally, the toner image on the paper sheet is fixed by a fixing apparatus.
One type of the fixing apparatus provided in the electronic photographing apparatus is a heat roller type fixing apparatus.
According to the heat roller type fixing apparatus, a heat roller is heated by heating means, and a recording paper sheet on which an image has been transferred is passed between a heat roller and a pressing roller. At this time, the paper sheet is pressed by the pressing roller and the toner image bearing surface of the paper sheet is brought into contact with the heat roller. Thus, the toner image on the toner image bearing surface of the paper sheet is heated, pressed on the paper sheet, and fixed.
The heat roller type fixing apparatus is constituted by providing the heat roller, pressing roller and heating means in the body of the apparatus.
The heat roller is constituted by coating the surface of an aluminum cylinder with a synthetic resin material having a good mold release property and heat resistance property. The heat roller is forcibly driven by a rotary drive device. The pressing roller is formed of an elastic material having a heat insulation property and heat resistance property. The pressing roller is formed of the elastic material in order to bring the recording paper sheet into stable contact with the heat roller. The pressing roller is supported on a roller shaft. The roller shaft is made of a metal. This is because the use of metal increases a wear resistance of the roller shaft.
The roller shaft is rotatably supported on plain bearings which are made of a synthetic resin with high wear resistance. The synthetic resin is inexpensive and light. The lower portions of the plain bearings are supported by compression coil springs. The pressing roller is urged towards the heat roller by the plain bearings which are pushed by the springs. Thus, the pressing roller presses the recording paper sheet on the heat roller.
When the heat roller is forcibly rotated, the rotation is transmitted to the pressing roller by friction with the heat roller and paper sheet. As a result, both rollers rotate to convey the paper sheet and at the same time the toner image on the paper sheet is fixed.
The conventional heat roller type fixing apparatus, however, has the following problem:
The rotation of the pressing roller is delayed with respect to the rotation of the heat roller. Consequently, a slip due to the difference in rotational speed occurs between the pressing roller and the heat roller. Owing to the slip, the recording paper sheet may also slip over the heat roller, and the toner image transferred on the paper sheet may be distorted.